pc: simplify MachineClass::get_hotplug_handler handling

pc: simplify MachineClass::get_hotplug_handler handling

By default MachineClass::get_hotplug_handler is NULL and concrete board
should set it to it's own handler.
Considering there isn't any default handler, drop saving empty
MachineClass::get_hotplug_handler in child class and make PC code
consistent with spapr/s390x boards.

We can bring this back when actual usecase surfaces and do it
consistently across boards that use get_hotplug_handler().

Suggested-by: David Gibson <[email protected]>
Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Message-id: 1525691524[email protected]
Signed-off-by: Peter Maydell <[email protected]>